Vitamin A is the precursor of at least two critical metabolites in vertebrate biology, 11-cis-retinal, the visual chromophore and retinoic acid, a ligand for nuclear receptors. Absorption of dietary vitamin A and its uptake by targeted body tissues depends on specialized binding proteins, transporters, and enzymes. Key components of the involved metabolic machinery are lecithin: retinol acyltransferase (LRAT), an enzyme that catalyzes the formation of retinyl esters. The physiological role of LRAT has attracted scientific and clinical interest because this enzyme is essential for maintaining systemic vitamin A homeostasis and regenerating visual chromophore. LRAT is also attractive targets for the development of a tissue-specific drug delivery system. Yet, progress in understanding the biochemistry of vitamin A uptake and homeostasis is hindered by a shortage of data regarding the molecular basis of these processes. We have a longstanding research interest focused on understanding LRAT's catalytic and physiological action as well as a mechanism that governs cellular retinoid uptake. Through this application, we propose to elucidate the fundamental molecular mechanism of vitamin A processing by LRAT. In Aim 1, we will delineate the molecular adaptations that discriminate LRAT from related enzymes to confer its vitamin A specificity. We also will identify the LRAT-specific retinoid binding domain and key residues involved in the substrate-protein interaction. Specific Aim 2 will focus on determining the molecular mechanism for LRAT's acyltransferase activity. By solving and analyzing crystal structures of HRASLS/LRAT chimeric proteins, we will discover the critical molecular adaptations that led to the acquisition of acyltransferase activity. We will define these changes at several levels, including the general topology of the enzyme, adjustments of the phospholipid binding mode, and the mechanism of water exclusion from the enzyme's active site. Finally, in specific Aim 3, we will examine the retinoid specificity of the enzyme. We will analyze the structures of the chimeric enzymes in their retinoid-bound states to obtain detailed information regarding the exact location and organization of the vitamin A binding site. Together, these studies will contribute novel and comprehensive knowledge, filling the gap in our understanding of the vitamin A metabolism and production of the visual chromophore. Dissecting molecular mechanisms underlying retinoid esterification will pave the way for more efficient treatment of human retinal degenerative diseases.